The reasoning behind the new study is that caffeine’s impact on musculature may reach significance only in large muscle groups or when used over time. In long efforts like endurance training, caffeine is unanimously an ergogenic aid. However, in power and strength trials the effects are less clear.

If caffeine's primary area of influence is the nervous system, then it stands to reason that muscles with a larger number of motor units will be more influenced by it. The authors addressed this question and hoped to clear up some of the past confusion in the research.

Study Design

Sixteen recreationally trained young men participated in this study. They were chosen in part because none of them were heavy habitual caffeine consumers. On average they consumed 300mg of caffeine or less each day, which amounts to a couple cups of coffee.

During the test, each of the subjects consumed 6mg of caffeine per kilogram of bodyweight. For a 200lb man this would be about 540mg of caffeine. On a different occasion, they consumed a placebo instead. The order of the tests was randomized. Thirty minutes after caffeine ingestion, the maximum voluntary contraction of each participant was recorded for the quads, calves, biceps, and wrist flexors.

Results

Caffeine significantly improved peak torque for all of the muscle groups in the upper and lower body. Although the motor unit recruitment didn’t appear to be significant in the prior study mentioned above, in this new study it seemed to work even for the biceps and wrist flexors, which were the smallest of the studied muscles. There was a trend towards larger muscles gaining a greater benefit from the caffeine, but it did not reach significance.

The results for improvement when using caffeine over placebo were as follows, in order from largest muscle group to smallest.

Quads: 13.7%

Calves: 11.2%

Biceps: 9.1%

Wrist Flexors: 6.3%

These results suggest that my hypothesis in the prior article was correct. Whatever the exact cause of the improvement in strength performance from caffeine ingestion, a small difference adds up the more muscle fibers you use.